Solar technician cleaning an array on a flat roof. A perfect place for dust to settle and block the sun.

Soiling is not always obvious until you start cleaning, but it can reduce PV module output significantly.

A failed attempt at keeping pigeons from nesting under a PV array: Not only did weeds grow from the droppings, shading the array, but some of the screen has come loose and contributes to shading, too.

Bird spikes, placed on the rack before the modules are mounted, effectively keep birds from roosting on and soiling modules.

This module suffers from delamination between the glass encapsulant and several PV cells. The cell to the upper left also appears to have moisture-induced corrosion.

Sometimes, module defects, such as this broken cell, can be spotted before the module is installed.

The bus bars and cell interconnects on this module have high electrical resistance, leading to extreme hot spots and actual burns on the bus bars, interconnects, and even on the polymer back sheet (not shown).

Cracked glass encapsulant allows water intrusion, which can cause corrosion and even a shock hazard.

An infrared camera can be a handy tool in troubleshooting PV module problems.

Advanced

Several situations can affect a PV system’s output, and new troubleshooting tools are available to help identify system losses and take some of the guesswork out of pinpointing the problems.

Production-based incentives, which pay system owners based on the amount of energy (kWh) their systems generate, make keeping an eye on system performance even more important than for other incentive programs. Being aware of common array problems, knowing how to maintain the system, and understanding how to evaluate performance (and how to troubleshoot) are vital to keeping performance and incentive payouts at their peak.

Potential Array Problems

While PV systems have no moving parts (compared to wind and microhydro systems) and can be extremely reliable, it does not mean they do not have potential performance problems, which can stem from external and internal issues.

External issues, such as shade from growing trees and module soiling (dust or soot from local air pollution), are common problems that can reduce energy output significantly. Studies on module soiling show an average annual energy loss of 5% for arrays that are not periodically cleaned. These types of problems are usually easily solved by intermittently trimming vegetation and cleaning arrays.

Impact to PV systems from critters is another external issue, but one that takes a little more consideration to fix. Wires might be damaged by rodents chewing on them; modules soiled by birds pooping on them; or cells shaded by weeds sprouting between the module frames from dirt and/or bird “fertilizer” beneath the array.

The fix-it for stopping critters in their tracks is to install rodent barriers and/or bird spikes. Many installers are tackling this problem preemptively, including some kind of screen or wire that keeps critters out but allows air to flow beneath the array.

Internal problems, such as module/cell damage, can also reduce system output. Sometimes these problems are easy to spot, but often they are not.

Visually inspecting the PV array once a year is a good idea. Look for cracks in the glass, brown/burn spots on both the front and the back of the modules, burnt solder joints on the cell “grid,” and signs of delamination and cell damage.

System Evaluation & Troubleshooting Tools

If the modules pass a visual inspection, that’s a good first step. In many cases, though, module/cell damage is invisible, so it is important to know how to evaluate your system’s performance, which can alert you to problems that may be brewing.

System owners often use the inverter’s built-in meter as the primary indicator of system performance. During a clear, sunny day around noon, you can check the system’s output (in watts) and compare it to the array size, derated to account for system inefficiencies. System efficiency typically ranges from 70% to 80%. This derate accounts for power losses due to module heating, dust, inverter efficiency, wiring voltage drop, module production tolerance, and module mismatch. For example, a 4,000-watt PV array would be expected to generate 70% to 80% of that value, or 2,800 to 3,200 watts. What action do you need to take if the meter reading is significantly lower than expected?